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This is a large taxon of insects; some estimates of the species numbers suggest well over 10000 world-wide.[1] Males are easily recognized by their plumoseantennae. Adults are known by a variety of vague and inconsistent common names, largely by confusion with other insects. For example, chironomids are known as "lake flies" in parts of Canada and Lake Winnebago, Wisconsin, but "bay flies" in the areas near the bay of Green Bay, Wisconsin. They are called "sand flies", "muckleheads",[2] "muffleheads",[3] "Canadian soldiers",[4] or "American soldiers"[5] in various regions of the Great Lakes area. They have been called "blind mosquitoes" or "chizzywinks" in Florida.[6] However, they are not mosquitoes of any sort, and the term "sandflies" generally refers to various species of biting flies unrelated to the Chironomidae.

The biodiversity of the Chironomidae often goes unnoticed because they are notoriously difficult to identify and ecologists usually record them by species groups. Each morphologically distinct group comprises a number of morphologically identical (sibling) species that can only be identified by rearing adult males or by cytogenetic analysis of the polytene chromosomes. Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881. They form through repeated rounds of DNA replication without cell division, resulting in characteristic light and dark banding patterns which can be used to identify inversions and deletions which allow species identification.

Larval stages of the Chironomidae can be found in almost any aquatic or semiaquatic habitat, including treeholes, bromeliads, rotting vegetation, soil, and in sewage and artificial containers. They form an important fraction of the macrozoobenthos of most freshwater ecosystems. They are often associated with degraded or low-biodiversity ecosystems because some species have adapted to virtually anoxic conditions and are dominant in polluted waters. Larvae of some species are bright red in color due to a hemoglobin analog; these are often known as "bloodworms".[7] Their ability to capture oxygen is further increased by their making undulating movements.[8]

Many reference sources in the past century or so have repeated the assertion that the Chironomidae do not feed as adults, but an increasing body of evidence contradicts this view. Adults of many species do, in fact, feed. The natural foods reported include fresh fly droppings, nectar, pollen, honeydew, and various sugar-rich materials.[1]

The question whether feeding is of practical importance has by now been clearly settled for some Chironomus species, at least; specimens that had fed on sucrose flew far longer than starved specimens, and starved females longer than starved males, which suggested they had eclosed with larger reserves of energy than the males. Some authors suggest the females and males apply the resources obtained in feeding differently. Males expend the extra energy on flight, while females use their food resources to achieve longer lifespans. The respective strategies should be compatible with maximal probability of successful mating and reproduction in those species that do not mate immediately after eclosion, and in particular in species that have more than one egg mass maturing, the less developed masses being oviposited after a delay. Such variables also would be relevant to species that exploit wind for dispersal, laying eggs at intervals. Chironomids that feed on nectar or pollen may well be of importance as pollinators, but current evidence on such points is largely anecdotal. However, the content of protein and other nutrients in pollen, in comparison to nectar, might well contribute to the females' reproductive capacities.[1]

Adults can be pests when they emerge in large numbers. They can damage paint, brick, and other surfaces with their droppings. When large numbers of adults die, they can build up into malodorous piles. They can provoke allergic reactions in sensitive individuals.[9]

The Chironomidae are important as indicator organisms, i.e., the presence, absence, or quantities of various species in a body of water can indicate whether pollutants are present. Also, their fossils are widely used by palaeolimnologists as indicators of past environmental changes, including past climatic variability.[10] Contemporary specimens are used by forensic entomologists as medico-legal markers for the postmortem interval assessment.[11]

A number of chironomid species inhabit marine habitats. Midges of the genus Clunio are found in the intertidal zone, where they have adjusted their entire life cycle to the rhythm of the tides. This made the species Clunio marinus an important model species for research in the field of chronobiology.[12]

Anhydrobiosis is the ability of an organism to survive in the dry state. Anhydrobiotic larvae of the African chironomid Polypedilum vanderplanki can withstand prolonged complete desiccation (reviewed by Cornette and Kikawada[13]). These larvae can also withstand other external stresses including ionizing radiation.[14] The effects of anhydrobiosis, gamma ray and heavy-ion irradiation on the nuclear DNA and gene expression of these larvae were studied by Gusev et al.[14] They found that larval DNA becomes severely fragmented both upon anhydrobiosis and irradiation, and that these breaks are later repaired during rehydration or upon recovery from irradiation. An analysis of gene expression and antioxidant activity suggested the importance of removal of reactive oxygen species as well as the removal of DNA damages by repair enzymes. Expression of genes encoding DNA repair enzymes increased upon entering anhydrobiosis or upon exposure to radiation, and these increases indicated that when DNA damages occurred, they were subsequently repaired. In particular, expression of the Rad51 gene was substantially up-regulated following irradiation and during rehydration.[14] The Rad51 protein plays a key role in homologous recombination, a process required for the accurate repair of DNA double-strand breaks.